1. Field of the Invention
The present invention relates to a method and apparatus for preventing, decomposing and removing emissions from an internal combustion engine. More particularly, the invention relates to a method and apparatus for preventing, decomposing and removing emissions under both cold start conditions and during continuous operation of an internal combustion engine.
2. Background of the Related Art
The exhaust gases from boilers, smelters, diesel generators, jet engines, gas turbine engines, automobiles, and trucks contain considerable amounts of nitrogen oxide compounds (NO.sub.x), unburned hydrocarbons (HCs) and carbon monoxide (CO). Nitrogen oxide, though thermodynamically unstable, does not spontaneously decompose in the absence of a catalyst. For emissions from engine operations usually near-stoichiometric air/fuel ratios, the reduction of NO.sub.x by CO and residual hydrocarbons is achieved by what is often called a "three way catalyst" (TWC). No satisfactory catalyst system exists, however, for NO.sub.x, HCs and CO abatement in exhaust gases from internal combustion engines which contain an excess of fuel under cold start conditions and an excess of oxygen under continuous operation conditions.
TWC catalysts arc currently formulated and designed to be effective over a specific operating range of both lean and rich fuel/air conditions and a specific operating temperature range. These particulate catalyst compositions enable optimization of the conversion of HCs, CO, and NO.sub.x. This purification of the exhaust stream by the catalytic converter is dependent upon the temperature of the exhaust gas and the catalytic converter works optimally at an elevated catalyst temperature, generally at or above about 300.degree. C. The time period between when the exhaust emissions begin (i.e., "cold start"), until the time when the substrate heats up to a light-off temperature, is generally referred to as the light-off time. Light-off temperature is generally defined as the catalyst temperature at which fifty percent (50%) of the emissions from the engine are being converted as they pass through the catalyst.
The conventional method of heating the catalytic converter is to heat the catalyst by contact with high temperature exhaust gases from the engine. This heating, in conjunction with the exothermic nature of the oxidation reactions occurring at the catalyst, will bring the catalyst to light-off temperature. However, until the light-off temperature is reached, the exhaust gases pass through the catalytic converter relatively unchanged. In addition, the composition of the engine exhaust gas changes as the engine temperature increase from a cold start temperature to an operating temperature, and the typical TWC is designed to work best with the exhaust gas composition that is present at normal elevated engine operating temperatures.
Selective Catalytic Reduction (SCR) is one measure that is being explored with regard to NO.sub.x reduction. Ammonia is injected into the exhaust gases to react with NO.sub.x over a catalyst to form nitrogen and water. Three types of catalysts have been used, including base metal systems, noble metal systems and zeolite systems. The noble metal catalysts operate in a low temperature regime (240-270.degree. C.), but are inhibited by the presence of SO.sub.2. The base metal catalysts, such as vanadium pentoxide and titanium dioxide, operate in the intermediate temperature range (310.degree.-400.degree. C.), but at high temperatures they tend to promote oxidation of SO.sub.2 to SO.sub.3. The zeolites can withstand temperatures up to 600.degree. C. and, when impregnated with a base metal, have an even wider range of operating temperatures.
SCR systems with ammonia as a reductant have been successfully employed to yield NO.sub.x reduction efficiencies of more than 80% in large natural gas fired turbine engines, and lean burn diesel engines (that run rich in oxygen). However, problems arise due to a strong dependence of the ammonia reaction and the catalyst life on exhaust gas temperature. The requirement of ammonia itself presents several problems. Ammonia is a toxic gas and is included in the EPA's list of extremely hazardous substances. The most critical aspect of SCR systems include safe ammonia handling, control of reactor temperature at all operating conditions, control of exhaust temperatures, and a dynamic ammonia dosage control system to maintain an optimum ammonia/NO.sub.x mole ratio under varying engine speed and load conditions. Some ammonia slip is unavoidable due to imperfect distribution of the reacting gases.
Selective Catalytic Reduction with hydrocarbons is another measure used to reduce NOx emissions. NO.sub.x can be selectively reduced by a variety of organic compounds (e.g. alkanes, olefins, alcohols) over several catalysts under excess O.sub.2 conditions. The injection of diesel or methanol has been explored in heavy-duty stationary diesel engines to supplement the HCs in the exhaust stream. However, the conversion efficiency was significantly reduced outside the narrow temperature range of 300.degree. C. to 400.degree. C. In addition, this technique suffers from the same problems as those of SCR with ammonia, such as HC-slippage over the catalyst, transportation and on-site bulk storage of hydrocarbons, and possible accidental release of the HCs into the atmosphere. The partial oxidation of hydrocarbons also releases undesirable CO, unburned HCs and particulates.
Another method to decrease NOx proposes using combustion at excessively lean air-fuel ratios to provide a combination of a decrease in NOx emissions and an increase in fuel economy in a lean burn engine. However, when a vehicle engine is operated at air-fuel ratios lean enough to decrease NOx, the combustion approaches a misfire limit, and driveability is impaired. To prevent this, an improvement has been proposed, wherein turbulences are generated within an engine cylinder so that the burning velocity is increased to thereby shift the misfire limit to the lean side. However, if the turbulences arc excessive and the flow velocity becomes too high, formation of a flame core and propagation of the flame in an early period of combustion will be obstructed. Another improvement has been proposed, where the air-fuel ratio distribution within an engine cylinder is controlled so that rich air-fuel mixtures are formed only in a region close to the ignition plug to produce easy ignition. However, when the misfire limit is shifted to the lean side, the effect on the NOx concentration also is decreased.
Another proposed method to decrease NOx provides an engine with air-fuel ratios slightly closer to the stoichiometric air-fuel ratio than the misfire limit and then purifies the insufficiently decreased NOx by using a zeolite-type lean NOx catalyst. This method has the potential to provide a clean system that also has good fuel economy. However, since the lean NOx catalyst can operate only under oxidizing exhaust gas conditions and is usually exposed to high temperatures, it is difficult to obtain both a sufficiently high NOx conversion by the lean NOx catalyst and a durable catalyst.
Lean burn engines, including lean burn gasoline engines and diesel engines, produce an exhaust gas that has an excess of oxygen (O.sub.2), that is, they are operated under oxidizing gas conditions. The leaner the air-fuel ratio, the greater is the concentration of O.sub.2 included in the exhaust gas. A catalyst which reduces NOx under oxidizing gas conditions is defined as a lean NOx catalyst, which is usually composed of a noble metal-type catalyst or a zeolite-type catalyst. At temperatures above 350.degree. C., NOx reduction occurs primarily by reaction with HC, while at low temperatures below 250.degree.-350.degree. C., NOx reduction occurs primarily by reaction with hydrogen (H.sub.2), wherein NOx purification by H.sub.2 is possible.
However, since the lean NOx catalyst is usually installed in or near an engine exhaust manifold in a conventional exhaust system, the temperature to which the catalyst is exposed is as high as 800.degree.-900.degree. C. Further, since the lean bum engine is operated at above stoichiometric air-fuel ratios, almost no H.sub.2 remains in the exhaust gas. Therefore, the NOx reduction characteristic of a lean NOx catalyst at low temperatures below 250.degree.-350.degree. C. has not been used in a conventional lean burn gasoline engine or diesel engine.
Therefore, there is a need for a cost effective, fuel efficient method and apparatus for decreasing NO.sub.x, HCs and CO emissions from internal combustion engines. It would be desirable if the method and apparatus removed HCs, CO and NOx during cold start as well as during continuous operation of an internal combustion engine. It would be further desirable if the method and apparatus could be implemented on existing engines and did not require large inventories of chemicals.